Prior Art
Conventionally, from the viewpoints of, for example, light weight, grain, and processability, wood is widely used for buildings such as housings.
However, a major drawback of the wood in use as a building material is high combustibility, so that various processes for introducing a fire-retardant property to wood have been and are in the development.
In conjunction therewith, also application fields thereof have been and are widened.
For example, since the Building Standard law regarding doorframes was revised (1990), use of wood has been permitted for opening portions. As such, development for fire retardation processes has been advancing to satisfy requirements of the standard regarding a flame-penetration resistance, specifically requiring 60-minute resistance for first-class materials and 20-minute resistance for second-class materials.
At present, by way of an example of a fire retardation process, there is a method in which wood is impregnated with a fire retardant, such as a water-soluble inorganic compound composed of boric acid, borax, and the like.
According to the process of impregnation with the fire retardant, the wood tissue can be physically penetrated (impregnated) with chemicals such as borax and boric acid by decompression or compression (for example, as in steps shown in FIG. 4, drying step→decompression step→decompression impregnation step→compression impregnation step→drying step). Thus, the processing steps are not complicated, so that the process is relatively popularly employed.
For example, in the process, the wood is first dried to a predetermined moisture content.
Subsequently, the wood is degassed by being decompressed to a level permitting an incombustion agent to penetrate as in that state, and is compressed to allow the agent to further penetrate.
Thereafter, the wood is dried to cause incompatible liquid to be fixedly impregnated in the wood tissue.
Fire-retardant wood is manufactured in accordance with the processing steps described above.
In addition, it is necessary to satisfy the requirements of the standard for incompatibility testing (heating-rate testing: ISO 5660), which is required as a condition of, for example, fire-retardant material (fire-retardant wood) and an incompatible material (incombustible wood) defined in the present Building Standard law.
The standards are shown in Table 1.
According to the standards, the following requirements should be satisfied under testing conditions of a heating time of 20 minutes and an emission intensity of 50 kW/m2 for satisfying the requirements of the incombustible-wood standard.
1) A total heating value shall be 8 MJ/m2 or lower.
2) Neither crack nor pore passing through to a reverse side, which is detrimental to fire retardation, shall be present.
3) A maximum heating rate shall continue for 10 seconds or longer and shall not exceed 200 kW/m2.
In addition to the requirements of the standard for combustive exothermicity, requirements of the standard regarding gas toxicity, described below, should be satisfied.
4) In gas toxicity testing, an average deactivation time of a mouse shall be 6.8 minutes or longer.
However, according to the above-described conventional process that performs, for example, the decompression impregnation and the compression impregnation, and the like, while the requirements of the standard for fire-retardant wood and semi-incombustible wood can be satisfied, the requirements of the standard for incombustible wood cannot be satisfied.
More particularly, a bottleneck is imposed in that the condition 1), which requires the total calorific value of 8 MJ/m2 of lower, cannot be satisfied.
To satisfy the condition, a method in which an incombustibility treatment agent is impregnated as much as possible into the wood tissue has been considered effective and has been and is put into practice on a trial basis.
More specifically, from the viewpoint that it is effective to iterate the impregnation step to achieve impregnation with a large amount of the incombustion agent, drying (drying step) is firstly performed and decompression is then performed for impregnation (decompression impregnation step). Thereafter, a step of compression for impregnation (compression impregnation step) is performed, and further, the routine of decompression impregnation step→compression impregnation step is iterated (refer to FIG. 5).
However, even when the routine of the decompression impregnation step→compression impregnation step is iterated a certain number of times, while the incombustibility treatment agent is impregnated into a region at a certain depth of the wood tissue, the agent does not reach a region deeper than the region.
That is, a limitation inevitably takes place on the amount of the incombustibility treatment agent that can be impregnated into the wood tissue.
As described above, it is a present state that incombustible wood that satisfies the requirements of the standard for the incombustible wood according to the Building Standard law has not been provided to date.
Problems to be Solved by the Invention
The present invention is aimed to solve the problems described above.
Specifically, an object of the present invention is to provide a method of manufacturing incombustible wood enabling maximization of the amount of an incombustible agent that is to be impregnated into the wood.
Another object is to provide a method of manufacturing incombustible wood that completely satisfies incombustible-wood conditions required by the Building Standard law and that can be relatively easily manufactured.